Paper production involves the formation and dewatering of a web of cellulose fibers and optional fillers, and is generally performed in the presence of additives which can improve drainage and fines retention.
The use of aluminum compounds in papermaking is well known. Aluminum sulfate, alum, or papermaker's alum, Al.sub.2 (SO.sub.4).sub.3 .multidot.14H.sub.2 O, is frequently used in paper sizing which provides water resistance in the finished paper and as precipitating or fixing agents to complex added dyes. Sodium aluminate is also widely used in papermaking to permit addition of extra aluminum for sizing and to increase pH.
U.S. Pat. Nos. 4,385,961 (issued May 31, 1983), 4,388,150 (issued Jun. 14, 1983), 4,755,259 (issued Jul. 5, 1988), 4,961,825 (issued Oct. 9, 1990), and 4,980,025 (issued Dec. 25, 1990) disclose papermaking processes which involve use of a binder comprising a colloidal silica and a cationic polymer (starch or polyacrylamide). These patents caution against the use of other paper chemicals such as alum that can interfere with formation of the silica-cationic agglomerate. It is recommended to wait to add such agents until after the agglomerate is formed.
U.S. Pat. No. 4,643,801 (issued Feb. 17, 1987) discloses a papermaking process using a binder comprising a cationic starch and a combination of ( 1 ) an anionic water soluble high molecular weight vinylic polymer of molecular weight at least 500,000 and (2) dispersed silica of particle size ranging from 1-50 nanometers, optionally in the presence of active alumina such as alum, sodium aluminate, or polyaluminum hydroxychloride. There is no suggestion to combine cationic and anionic aluminum compounds to afford additional improvement to the process.
U.S. Pat. No. 4,902,382 (issued Feb. 20, 1990) discloses a process for producing a neutral paper at pH 6-9 which comprises adding to a paper stock slurry of filler and high yield pulp in order, a water soluble cationic aluminum salt, a cationic starch, and bentonite ultrafine clay, and after addition of the bentonite, or preferably, simultaneously with the bentonite, colloidal silica.
U.S. Pat. No. 4,964,954 (issued Oct. 23, 1990) discloses a process for the preparation of paper by forming and dewatering papermaking fibers on a wire at pH &gt;5 in the presence of a synthetic organic cationic polymeric retention agent, polyacrylamide or polyethyleneimine, an anionic inorganic colloid, especially silica sols containing aluminum, and a basic polyaluminum compound with at least 4 aluminum atoms per ion, preferably 10, with a ratio of polyaluminum compound to inorganic colloid of 0.01 to 3: 1.
U.S. Pat. No. 5,127,994 (issued Jul. 7, 1992) discloses a process for the production of paper by forming and dewatering a suspension of cellulose containing fibers and optional fillers on a wire in the presence of an aluminum compound, such as alum, polyaluminum compounds, aluminates, aluminum chloride and aluminum nitrate, a cationic polymeric retention agent, preferably cationic starch or cationic polyacrylamide, and a polymeric silicic acid prepared by the acidification of alkali metal silicate having a specific surface area of at least 1050m.sup.2 /g.
European Patent 0 357574 (published Jul. 3, 1990) discloses a process for the production of paper by forming and dewatering a suspension of cellulose containing fibers on a wire in the presence of an anionic inorganic colloid, a cationic synthetic polymer and an aluminate.
While cationic and anionic aluminum compounds have been used individually to improve various aspects of the papermaking process, the combination of these has not been recognized as providing an improvement in drainage and fines retention that is greater than the individual contribution of either aluminum compound.